Following discovery of oxide superconducting materials having a relatively high critical temperature (Tc) such as LiTi2O3, Ba(Bi, Pb)O3, and (Ba, K)BiO3, there have lately been developed copper oxide superconducting materials one after another, such as (La, Sr)2 CuO4, REBa2Cu3O7 (RE: rare earth element), Bi2Sr2Ca2Cu3O10, Ti2Ba2Ca2Cu3O10, and HgBa2Ca2Cu3O8 having still higher critical temperatures.
Incidentally, it has been known that although a superconducting material having a higher critical current density in comparison with an ordinary conducting material can pass a large electric current without loss as described above, there is a risk of the superconducting material being destroyed depending upon its strength in the case where such a large electric current is passed therethrough because a large electromagnetic force acts on superconductors.
Further, as a result of recent improvement in the characteristics of a high temperature bulk superconductor (particularly, a copper oxide superconductor) and recent trends for larger sizes thereof, the magnitude of a magnetic field that can be trapped in a bulk superconductor has increased by leaps and bounds. Such an increase in the magnitude of the magnetic field is accompanied by an increase in the electromagnetic force acting on the bulk superconductors, so that there has lately arisen a problem in that restriction is inevitably imposed on a trapped magnetic field depending on the strength of a bulk superconductor. Accordingly, for enhancement in performance of a bulk superconducting magnet utilizing a trapped magnetic field, it has become important to enhance the mechanical properties of the bulk superconductors rather than to further enhance the superconducting properties.
Accordingly, the inventor, et al. have previously proposed high-temperature bulk superconductors, having a considerably high mechanical strength, as follows:    a) an “oxide superconductor” (refer to JP, 3144675, B) comprising an oxide bulk superconductor produced by a melt process and having a resin-impregnated layer of an epoxy resin, and so forth (resin impregnated layer impregnated with resin through microcracks and voids unavoidably included therein in a process of producing an oxide bulk superconductor in the state of a pseudo single crystal in an atmosphere under a reduced pressure);    b) an “oxide superconductor” (refer to JP, 3144675, B) comprising an oxide bulk superconductor having a resin-impregnated layer and produced by a melt method, wherein the oxide bulk superconductor contains at most 40% by weight of Ag;    c) an “oxide superconductor” (refer to JP, 3100370, B) comprising an oxide bulk superconductor having a resin-impregnated layer and the outer surface thereof covered with a resin layer dispersedly incorporating a filler material of a small linear thermal expansion coefficient, such as quartz, calcium carbonate, alumina, alumina hydrate, glass, talc, calcined gypsum, and so forth, produced by a melt process;    d) an “oxide superconductor” (refer to JP, 3100375, B) comprising an oxide bulk superconductor having “an adhesively covering layer of resign-impregnated fabric” on the outer surface thereof and a resin-impregnated layer in a surface portion thereof, produced by a melt process; and so forth.
The above-described oxide superconductors (high-temperature bulk superconductors) with such a treatment of forming the resin-impregnated layer, applied thereto, have an excellent mechanical strength and consequently, have excellent characteristics in that they are capable of ensuring a high trapped magnetic field (high trapped magnetic field with the magnitude thereof enhanced to the extent in excess of 10 T in terms of magnetic flux density), which has not been seen before, and besides, deterioration of the trapped magnetic field is small, even after thermal cycles of cooling and warming, and electromagnetic hysteresis of electromagnetic force repeatedly applied thereto. As a result of further perusal thereof by the inventor, et al, however, it has become apparent that even those high-strength high-temperature bulk superconductors have the following problems.
More specifically, with a superconducting magnet made of the high-temperature bulk superconductor, there is normally adopted a magnetizing method of applying a magnetizing treatment to the high-temperature bulk superconductor placed in an external magnetic field higher than a trapped magnetic field aimed as a target while cooling the same with a gas evolved from liquid helium, and causing a magnetic field to be sufficiently trapped by gradually lowering the external magnetic field from the state described. In such a case, there will not occur a situation such that the high-temperature bulk superconductor reinforced by resin-impregnation, or the like is unable to withstand Lorentz force when an external magnetic field with a magnetic flux density in excess of 10 T is applied, as with the case of a conventional high-temperature bulk superconductor without reinforcement by resin-impregnation, or the like, thereby resulting in destruction. In the case of applying the external magnetic field with the magnetic flux density in excess of 10 T, as never experienced before, a phenomenon (quenching phenomenon) wherein a superconducting state is broken by generation of heat due to an avalanche-like movement of magnetic flux lines, called the flux jump, is prone to occur.
The higher a magnetic field becomes, the more often such a phenomenon, as observed when applying a magnetizing treatment to the high-temperature bulk superconductor, is prone to occur, and in the case of a magnetic field with a magnetic flux density in excess of 10 T, it has been found extremely difficult to magnetize even the high-temperature bulk superconductor with the reinforcing treatment applied thereto due to the above-described phenomenon.
That is, the inventors have found through experiments on the high-temperature bulk superconductor reinforced by resin-impregnation that in order to attain a higher trapped magnetic field in the high-temperature bulk superconductor, there is the need for avoiding adverse effects of generation of heat, due to the flux jump, in addition to the need for enhancement in mechanical strength of the high-temperature bulk superconductor.